Which term indicates a gradual decrease in volume and speed?

The correct term for a gradual decrease in volume and speed is smorzando. This Italian term translates to "dying away" and is used in music to indicate that the performer should gradually decrease both the dynamics and the tempo. Essentially, the music not only becomes softer but also slows down, creating a sense of fading away.

In contrast, the other terms listed have different meanings. Stringendo refers to a quickening of speed, often accompanied by an increase in intensity. Sostenuto means to sustain a note or passage longer than its usual value, creating a feeling of steadiness rather than a decrease. Sonoro signifies a rich, full sound, but does not imply any changes in volume or speed. Understanding these nuances in terminology is essential for interpreting and performing music accurately.